1. Field of the Invention
The present invention relates to a thin high-frequency transmission line for transmission of a high-frequency signal and an electronic device including the high-frequency transmission line.
2. Description of the Related Art
Typical examples of a high-frequency line for transmission of a high-frequency signal according to the related art include a coaxial cable. The coaxial cable includes a center conductor (signal conductor) configured to extend in one direction (configured to extend in the signal transmission direction), and a shield conductor provided concentrically along the outer peripheral surface of the center conductor.
In recent years, high-frequency devices including mobile communication terminals have been reduced in size and thickness, and a space for arrangement of the coaxial cable may not be secured in a terminal housing. In addition, the coaxial cable is hard, and thus not easily curved or bent to be routed.
Use of high-frequency transmission lines described in International Publication No. 2011/007660 and Japanese Registered Utility Model No. 3173143 is drawing attention to solve a problem that occurs when such a coaxial cable is utilized. The high-frequency transmission line is larger in width than the coaxial cable but can be thinned, and therefore is particularly useful in the case where there is only a small clearance in the terminal housing. In addition, the high-frequency transmission line has a flexible dielectric element as the base material, and thus is flexible and can be easily curved and bent to be routed.
The high-frequency transmission lines described in International Publication No. 2011/007660 and Japanese Registered Utility Model No. 3173143 have a tri-plate strip line structure as the basic structure.
The high-frequency transmission lines described in International Publication No. 2011/007660 and Japanese Registered Utility Model No. 3173143 include a flat-plate dielectric element that is flexible and insulating. The dielectric element has an elongated shape that extends straight. A second ground conductor is disposed on a second surface orthogonal to the thickness direction of the dielectric element. The second ground conductor has a so-called solidly filled conductor pattern in which the second surface of the base material sheet is covered generally entirely. A first ground conductor is disposed on a first surface of the base material sheet opposite to the second surface. The first ground conductor includes elongated conductors configured to extend along the longitudinal direction at both ends in the width direction orthogonal to the longitudinal direction and the thickness direction. The two elongated conductors are connected to each other by bridge conductors disposed at predetermined intervals along the longitudinal direction and configured to extend in the width direction. Thus, the first ground conductor is shaped such that opening portions with a predetermined opening length are formed and arranged along the longitudinal direction.
A signal conductor with a predetermined width and a predetermined thickness is formed in the middle of the dielectric element in the thickness direction. The signal conductor is elongated to extend in the direction parallel with the elongated conductor portions of the first ground conductor and the second ground conductor. The signal conductor is formed generally in the center of the dielectric element in the width direction.
When the thus configured high-frequency transmission line is viewed in plan (seen from a direction orthogonal to the first surface and the second surface), the signal conductor is disposed so as to overlap the first ground conductor only at the bridge conductors and be provided in the opening portions in other regions.
With such a shape, the high-frequency transmission lines having predetermined transmission characteristics can be thinned, and can be easily curved and bent to be routed. The terms “curve” and “bend” as used herein refer to three-dimensional deformation that causes the entire flat-plate surface of the high-frequency transmission lines not to be present on an identical plane. In other words, the terms relate to causing a bend by a predetermined angle with respect to the flat-plate surface of the high-frequency transmission lines.
However, the high-frequency transmission lines structured as discussed above have the following problem. FIGS. 14A and 14B are each a graph illustrating the distribution characteristics of characteristic impedance for explaining the problem with the high-frequency transmission line structured in accordance with the related art.
In the high-frequency transmission line according to the related art, the first ground conductor includes a plurality of opening portions provided along the longitudinal direction. Thus, the first ground conductor and the signal conductor face each other along the thickness direction only at the locations of installation of the bridge conductors. Therefore, a C property (capacitive property) becomes highest at the positions of the bridge conductors along the longitudinal direction, and an L property (inductive property) becomes highest in the middle of the opening portions.
Since the bridge conductors are disposed at predetermined intervals as discussed above, the characteristic impedance of the high-frequency transmission line according to the related art is cyclically varied in accordance with the interval of installation of the bridge conductors. Setting is made such that a desired characteristic impedance is obtained for the overall length of the high-frequency transmission line.
The characteristic impedance is set with the high-frequency transmission line not curved. Thus, with the high-frequency transmission line not curved, as illustrated in FIG. 14A, the characteristic impedance is varied in accordance with the interval of installation of the bridge conductors, and a desired characteristic impedance Zo is obtained for the overall length. In this event, the amplitude ΔR0 of the real number of the characteristic impedance has a constant value.
In the case where the high-frequency transmission line is curved, however, the positional relationship between the signal conductor and the first ground conductor and the second ground conductor is varied at curved portions. In this case, the characteristic impedance at the curved portions is varied. Depending on the curved state, as illustrated in FIG. 14B, for example, the L property in an opening portion may be increased such that the amplitude ΔR0′ of the real number of the characteristic impedance at the curved portions is larger than the amplitude ΔR0 of the real number of the characteristic impedance at non-curved portions.
The characteristic impedance for the entire high-frequency transmission line greatly depends on the maximum value of the amplitude of the real number of the characteristic impedance. Thus, if the high-frequency transmission line is curved and the maximum value of the amplitude of the real number of the characteristic impedance is varied, the characteristic impedance for the entire high-frequency transmission line is also varied. For example, as illustrated in FIG. 14B, the characteristic impedance Z0′ of the entire high-frequency transmission line with curved portions is different from the characteristic impedance Z0 of the entire high-frequency transmission line which is not curved. Therefore, the transmission loss of an RF signal may be increased to degrade the transmission characteristics.